Field effect transistors can be used as single-molecule charge sensors to identify molecules. Such sensors may be operated at biologically relevant salt conditions. The Debye screening length of such salt solutions is in the range of about 0.3 nm to about 10 nm, which limits the sensing zone to a few nanometers outside the surface of the channel and often reduces signal levels to the limit of detectability. A work-around to this difficulty is to perform the biological reaction and the measurements in two different buffers—high salt and low salt, respectively. However, such an approach is not typically suitable for single-molecule sensors where buffer exchange is not possible.